The present invention relates to a method of filtering suspension and an apparatus for realizing that method. The present invention may be applied for filtering green liquor containing finely divided matter, generated, for example, in the chemical circulation of a cellulose pulp mill.
A significant subprocess in the manufacture of sulphate cellulose pulp is the recovery of cooking chemicals. Part of the regeneration is formed by the manufacture of white liquor by causticizing, so that lime milk and green liquor are allowed to react in order to form lime sludge and white liquor. The green liquor is generated when a chemical melt containing sodium chemicals is led from the bottom of the furnace of a soda recovery boiler to a separate dissolving vessel, in which the melt is dissolved in a dilute liquor. The most significant sodium chemicals of the green liquor are sodium carbonate and sodium sulphide. The green liquor also contains insoluble compounds, such as metal oxides, silicates, soot and other impurities. The dregs containing impurities must be discharged from chemical circulation, since the dregs otherwise concentrate in the chemical circulation and disturb the manufacture of white liquor by causticizing. The cleaning of green liquor is generally performed in clarifiers. When the mills become larger and the environmental regulations more stringent cleaning by sedimentation principle results in large apparatus, the diameters of which exceed 30 meters. Moreover, clarifiers are always liable to disturbances, so that green liquor that is insufficiently pure may result even when correctly dimensioned apparatus is used.
An alternative method is to clean green liquor by filtering. Since the dregs mainly consist of finely divided matter, the filterability of which is poor and which rapidly clog the filtering surface, this method employs a filtering apparatus which has a large surface area and is thus expensive. A cake containing fine particles must thus often be removed from the filtering surface so that the filter remains efficient. When such a cake is washed off, a very dilute sludge is obtained, which must be further concentrated in several process stages.
In order to improve the filterability of, for example, lime sludge obtained from the causticizing process, a filtration aid may be used, by means of which a filtering layer having good filtering capacity when filtering finely divided green liquor is formed on the surface of the filter element. This procedure, however, results in additive costs and in an increased amount of waste being transported to a landfill site.
The filterability of a suspension, which is difficult to filter, has also been suggested to be improved in such a way that the formation of a filter cake on the filtering surface is prevented by removing separated solids by causing a strong shear force on the suspension close to the filtering surface, which mixes solids back to the suspension to be filtered. The problem with this cross-flow filtration has been the need to use large liquid volumes to be circulated in order to generate sufficient velocity and turbulence to remove the separated solids from the filtering area.
The purpose of the present invention is to provide a filtering method and apparatus, by which the disadvantages discussed above may be eliminated. The present invention provides a simpler and more economical method and apparatus for filtering liquid-solids suspensions, especially those which have so far been difficult to filter. Although the background of the problem has been discussed above in view of green liquor and the invention is particularly applicable thereto, it is only an example and the invention may be applied for filtering other liquid-solids suspensions, such as kaolin sludge, clarified white liquor, soda liquor, bleaching effluent of cellulose pulp production, liquids containing pulp fines at pulp mills, and liquids containing metal hydroxides.
In the filtering method of the invention the suspension to be filtered is brought into contact with the filtering surface of the filter element in a manner such that due to the pressure difference across the filtering surface the filtrate flows through the filtering surface and the separated solids substantially remain in the suspension. It is a characteristic feature of the invention that the suspension to be filtered is caused to flow downwardly on the filtering surface, so that filtration takes place from the film flowing on the filtering surface.
An apparatus in accordance with the present invention comprises a plurality of filter elements; the filtrate flows through the filtering surface of each while the separated solids substantially remain in the suspension, so that the filter is provided with at least a structure for passing the suspension to be filtered to the filtering surfaces and for removing the filtrate. It is a characteristic feature of the invention that the structure for passing the suspension to be filtered are positioned in such a way that the suspension is brought to an upper portion of each filtering surface to flow downwardly on the filtering surface. According to one preferred embodiment a filter is provided with means for recirculating the non-filtered suspension to an upper portion of the filtering surface.
The present invention realizes cross-flow filtration in a novel manner, such that shear forces preventing generation of solids cake on the filtering surface are formed by passing suspension to be filtered down the filtering surface to flow due to the gravitational force during filtration. Due to a pressure difference between different sides of the filtering surface part, of the liquid is filtered from the falling film through the filtering surface.
A continuous downwardly flowing liquid film is most suitably generated by recirculating non-filtered suspension to an upper portion of the filtering surface and the suspension to be filtered is supplied to and mixed with the circulation flow. The generation of a uniform liquid film on the filtering surface requires rather large liquid volumes (flow rates), generally larger than the amount of liquid which typically flows into the apparatus, therefore the recirculation of the suspension is highly desirable for efficient filtration.
Filter elements according to the present invention are preferably in the form of lamellas, tubes and/or discs. Usually the filtrate is discharged through a filter channel inside the element, such as a tube. If desired, the suspension to be filtered may be passed into the element, so that the filtration takes place in an opposite direction.
The separated solids tend to accumulate on the filtering surface. By maintaining the flow velocity high enough the generation of the solids layer may be completely prevented, since separating solid particles are entrained with the falling liquid. If a solids layer appears to be generating, it may be prevented or removed by lowering the pressure difference across the filter surface for a short period of time (e.g. a few minutes) so that the flow through the filtering surface ends or slows down and the "grip" of the layer from the filtering surface loosens, and when the downward flow loosens it, it flows with the suspension. In order to remove the harmful solids cake the volume of flow in the falling film may be intensified locally (e.g. in one or more restricted areas at a time); the increase of the flow may be generated by means of one or more liquid jets, for example.
The attachment of solids on the filtering surface may also be prevented by vibration using known mechanical or acoustical vibrating equipment.
The suspension to be filtered is typically distributed on the filtering surface in the same way as in conventional falling film evaporators, such as those sold by A. Ahlstrom Corporation of Helsinki, Finland. The suspension to be filtered is fed to a liquid distribution tray--having a bottom with evenly spaced perforations (openings)--mounted above the filter surfaces, and flows from the tray evenly over the filter.
The pressure difference across the filtering surface may be generated by joining the filter to conventional vacuum generating apparatus, such as a vacuum pump. The filter elements may thereby be assembled in an atmospheric pressure vessel or completely open to the atmosphere.
A required pressure difference may alternatively or additionally be maintained by pressurizing a filter in a pressure vessel with gas. The gas may be provided in a closed circulation system. The applied gas may be inert or reactive relative to the suspension to be filtered. Reactive gas is added, when so desired, during the filtration in order to realize desired chemical reactions.
Gas which might penetrate the filtering surface may be separated from the filtrate either in the filter element itself by removing it from a separate conduit than the filtrate, or (more preferably) from above the liquid surface through the upper part of the element, or in a separate vessel outside and distinct from the filter.
The filtration may take place either continuously or in batches. In continuous filtration new suspension is supplied, and thickened suspension is discharged, continuously. If however, dregs as clean as possible are desired (which for green liquor means efficient alkali removal from the dregs), it is possible to perform the filtration in batches.
In batch processes non-filtered suspension is not discharged continuously, but the feed of the suspension is stopped during filtration and the suspension is allowed to be thickened by recirculating. Thereafter wash water is supplied to the filter and filtration continues. The wash water, which mixes with the dregs, removes impurities (e.g. alkali) from the dregs. At the same time the water penetrating the filtering surface washes the filtering surface improving the filtration capacity. The dregs are removed as sludge to be further treated and a new filtration cycle may be begun.
An even more efficient treatment of dregs is provided, if a dregs cake is formed for washing. The apparatus according to the invention may be provided with a separate sludge vessel. A recirculation pump draws the suspension to be recirculated from the sludge vessel (to which the sludge flows from the bottom of the filter). The process is initiated by filling the sludge vessel with virgin suspension to be filtered, whereafter the recirculation and filtration of the suspension is started. Sludge from the filter is not removed from the sludge vessel, but rather is allowed to thicken there, which takes place when liquid is removed as filtrate. When the solids content of the sludge is at the desired level, the bottom conduit of the filter is closed and the filter is allowed to be filled with sludge. The pressure difference across the filter surface still prevails in a full filter, whereby liquid infiltrates through the filtering surface, on which a solids cake is generated. After filtration the sludge remaining between the filter elements or otherwise in the filter housing is removed and brought back to the sludge vessel. When the bottom of the filter is closed, a dregs cake remains due to the pressure difference on the surface of the elements. The gas flowing through the cake and the filter element removes humidity, drying the dregs cake. The generated cake is removed by washing with water and blowing with gas to be passed for further treatment (washing and drying). This entire procedure provides for very efficient treatment of dregs by the formation of a dregs cake.
In connection with a batch process as described above it is possible to carry out dregs washing with very efficient displacement washing techniques. In the art it has been typically necessary to carry out the dregs washing in separate apparatus. Washing according to the invention takes place by filling the filter housing with water after the dregs cake is dried, and then by discharging the filtered water to a washing water vessel. The cake is finally removed, as above, by washing with water and blowing with gas and the dregs-water mixture is passed from the filter housing to further treatment. The dried cake may alternatively be removed from the open bottom of the filter housing simply by blowing with air or other gas.
Thus, if so desired, according to the invention the entire treatment of the material to be filtered (such as green liquor) may be performed in the same apparatus (filter housing).
In batch process it is possible to adjust the capacity by pressure difference or by extending the interval between the cycles.
According to one aspect of the present invention, a method of filtering a suspension of solids in a liquid, using a filter surface having first and second sides and a generally vertical orientation, is provided. The method comprises the steps of (a) Causing the suspension to flow in a falling film down the first side of the filter surface. (b) Providing a higher gas pressure on the first side of the filter surface than on the second side of the filter surface, so that filtrate passes through the filter surface to the second side of the filter surface, while solids remain in the liquid flowing down the filter surface first side. (c) Withdrawing the filtrate away from the filter surface. And, (d) withdrawing the liquid with suspended solids away from the filter surface.
Step (b) may be practiced by supplying gas under superatmospheric pressure to the first side of the filter surface and/or by applying a partial vacuum to the second side of the filter surface. Partial vacuum and gas pressurization may be provided by using a vessel having an inlet, outlet, gas volume, and a compressing device connected to the outlet, in which case step (b) is practiced by causing gas from the second side of the filter surface to pass into the vessel through the vessel inlet, to pass out of the vessel through the vessel outlet, to be compressed by the gas compressing device, and then to pass toward the first side of the filter surface. The filter surface has a top and a bottom and step (a) is typically practiced by recirculating the liquid from step (d) from adjacent the bottom to adjacent the top, to flow in a falling film down the filter surface. The gas that is supplied may be gas that is inert to the suspension, or chemically reacts with the suspension.
Then there may be the further step (e) of locally intensifying rate or volume of falling film flow of suspension over the first side of the filter surface so as to substantially preclude the formation of a solids cake on the filter surface. Step (e) may be practiced by directing one or more liquid jets toward the filter surface first side. Alternatively, the prevention of the formation of a solids cake may be practiced by at least occasionally vibrating the filter surface either mechanically or acoustically.
Steps (a) through (d) may be practiced continuously or in batches, and where the suspension being filtered is green liquor the ratio of the flow velocity V to the average filtrate flow velocity V.sub.s through the filter surface is at least 5:1 (preferably about 8:1). During batch filtration, in which there is continuous recirculation but no introduction of virgin suspension, the dregs may be washed in the filter housing, and/or the dregs may be removed dry by blowing air or other gas through the inside of the filter elements to force the dregs cake to the bottom of the filter vessel from which it can be removed by opening the bottom of the vessel.
According to another aspect of the invention, a method of cleaning green liquor using a filter surface having first and second sides and a generally vertical orientation, is provided. The method comprises the steps of: (a) Causing the green liquor to flow in a falling film down the first side of the filter surface. (b) Providing a higher gas pressure on the first side of the filter surface than on the second side of the filter surface, so that filtrate passes through the filter surface to the second side of the filter surface, while solids remain in the liquid flowing down the filter surface first side. (c) Withdrawing the filtrate away from the filter surface. And, (d) withdrawing the green liquor away from the filter surface.
According to another aspect of the present invention a filtering apparatus is provided comprising the following components: A housing. A plurality of filter elements generally vertically mounted in the housing, and including a top and a bottom. Each filter element comprising a filter surface with first and second sides. Means for providing a higher gas pressure on the first sides of the filter surfaces with respect to the second sides. A first conduit for withdrawing filtrate from the second side of at least one of the surfaces. A second conduit for withdrawing suspension liquid from adjacent the bottom of the elements. And, means for introducing suspension into said first sides of the surfaces to flow in a falling film down the first sides.
The apparatus may also comprise a sludge vessel connected to the second conduit external of the housing, and an open dregs vessel distinct from the sludge vessel connected to a bottom portion of the housing. Recirculating means are typically provided for recirculating suspension from the sludge vessel to the introducing means, and the introducing means may comprise a tray with perforated bottom mounted above the filter element. The introducing means may also or alternatively comprise at least one nozzle for directing suspension onto at least one of the filter surface first sides. When the housing is gas-tight gas pressure providing means comprise means for introducing gas under pressure into the housing. The gas pressure providing means may alternatively or additionally comprise means for applying a partial vacuum to the filter surface second sides. There may also be means for withdrawing gas from the second sides of the filter surfaces distinct from the first conduit, or for simultaneously withdrawing gas and filtrate through the first conduit.